1. Field
Communication systems may benefit from various aspects of emergency call handing. For example, wireless communication systems may benefit from clean up with respect to a fast return feature applied to an emergency call using circuit switched fallback, without a location area update.
2. Description of the Related Art
Circuit switched fallback (CSFB) in the long term evolution (LTE) of third generation partnership project (3GPP) can permit a user equipment (UE) to reuse circuit switched (CS) domain for a voice call. This can include an emergency (E911) call. In release 10 (R10), a so-called “fast return” feature can be used to permit the user equipment to revert back immediately to LTE after the CS call is completed or the connection to the mobile switching center (MSC) is released.
For a mobile originated call using circuit switched fallback on a normal call, not an emergency call, the user equipment is conventionally required to first check the location area (LA) of the second or third generation (2/3G) cell that it has connected to. The user equipment then first performs a location area update (LAU), if this LA is different from the one that the user equipment has stored. This location area update procedure can ensure that the home location register (HLR) has the current serving node address, such as the address of the mobile switching center (MSC). This location area update can also update the LA that is stored inside the user equipment and is sent to a mobility management entity (MME) when the user equipment returns back to LTE. The MME can check this LA to determine whether an SGs interface needs to be re-established or not. The SGs interface can be the reference point between the MME and MSC server (MSS).
For an emergency call using CSFB, the user equipment can immediately proceed with a circuit switched (CS) call using an international mobile subscriber identity (IMSI) without first performing a location area update procedure even when the LA of the serving cell is different from the one that the user equipment has stored internally. In this case, even if the user equipment is not known in this MSC/visitor location register (VLR), based on configuration, MSC can still proceed with call setup toward a public safety answering point (PSAP). This is to ensure the E911 call setup is not delayed. After the CS emergency call is completed, the user equipment can perform a location area update procedure. If a fast return feature is used, the user equipment can immediately be redirected back to LTE without location area update.
Without location area update, the user equipment is unknown in the serving MSC/VLR. Thus, the home location register no longer has the current serving node information. This can result in an issue with a mobile terminating location request (MT-LR) procedure initiated by PSAP. However, this can be solved by having the serving MSC initiate the location area update procedure on behalf of the user equipment at the beginning of the call. Thus, one way of addressing a circuit switched call that happened as the result of circuit switched fallback is for a mobile switching center to perform a location area update on behalf of a user equipment. This approach can make the system's knowledge of the UE's serving node inconsistent. MME knows that UE is still registered in the previous MSC. UE knows that LA has not changed, and so there was no location area update form UE's perspective. Thus, when the emergency call is over and UE returns to LTE with fast return functionality, without executing a location update, the UE will not re-register to the previous MSC, and the UE becomes unreachable for any mobile terminated activity.